1. Field of the Invention
The present invention relates to a coagulation treatment apparatus, a coagulation treatment method, a coagulant, a coagulant production apparatus and a coagulant production method. More particularly, the present invention relates to the coagulation treatment apparatus, the coagulation treatment method, the coagulant, the coagulant production apparatus and the coagulant production method for performing treatment of wastewater containing fine objects of removal.
2. Description of the Related Art
Presently, the diminishing of the amount of industrial waste, separate collection and recycling of industrial waste, and prevention of release of industrial waste are considered to be ecologically-important topics and business issues as society moves towards the 21st Century. Some types of industrial waste comprise various types of fluids containing objects of removal; i.e., substances to be removed.
Such fluids are known by a variety of expressions, such as sewage, drainage, and effluent. Fluids, such as water or chemicals, containing substances that are objects of removal, shall be hereinafter referred to as “wastewater.” The objects of removal are eliminated from wastewater by means of an expensive filtration system or a similar system. Wastewater is thereby recycled as a clean fluid, and the removed objects of removal or substances that cannot pass through the filtration system are disposed of as industrial waste. In particular, water is sent back to a natural setting, such as a river or sea, or recycled after being purified so as to meet environmental standards.
Adoption of such a filtration system is difficult because of costs incurred in constructing and running a filtration system, thus posing an environmental problem.
As can be seen from the above, wastewater treatment techniques are important in terms of recycling and prevention of environmental contamination, and immediate demand exists for a filtration system that incurs low initial and running costs.
By way of illustration, wastewater treatment as practiced in the field of semiconductors shall now be described. When a plate member formed, for example, from a metal, a semiconductor, or ceramic, is ground or abraded, an abrasion (or grinding) jig or the plate member is subject to a shower of a fluid, such as water, for preventing an increase in the temperature of the abrasion (or grinding) jig, which would otherwise be caused by friction, for improving lubricity, and for preventing adhesion of abrasion or grinding waste onto the plate member.
More specifically, in the process of dicing or back-grinding of plate-like semiconductor material; e.g., a semiconductor wafer, pure water is made to flow over the semiconductor wafer. In a dicing machine, a shower of pure water is made to flow over a semiconductor wafer, or pure water is squirted onto a dicing blade from a discharge nozzle in order to prevent an increase in the temperature of the blade or adhesion of dicing waste onto the semiconductor wafer. For the same reason, a flow of pure water is employed during an operation in which a semiconductor wafer is made thin by means of back-grinding.
Wastewater, which has mixed therein grinding or abrasion waste and is discharged from the dicing or back-grinding machine, is returned to a natural setting or recycled after having been purified through a filter. Alternatively, concentrated wastewater is recovered.
In a current process for manufacturing a semiconductor, wastewater, in which objects of removal (i.e., waste) primarily including Si are mixed, is disposed of according to either of two methods; i.e., a coagulating sedimentation method and a method, which employs a filter and a centrifugal separator in combination.
Under the coagulating sedimentation method, polyaluminum chloride (PAC) or aluminum sulfate (Al2(SO4)3) is mixed in the wastewater as a coagulant to generate a reaction product with Si and the wastewater is filtrated to remove this reaction product.
Under the method that employs a filter and a centrifugal separator in combination, the wastewater is filtrated, the concentrated wastewater is processed by the centrifugal separator to recover the silicon waste as sludge, and the clear water resulting from filtration of the wastewater is released to a natural setting or is recycled.
For example, as shown in FIG. 17, wastewater discharged during a dicing operation is collected into a raw water tank 201 and is sent by a pump 202 to a filtration unit 203. A ceramic-based or an organic-based filter F is provided in the filtration unit 203, and the filtrated water is delivered via a pipe 204 to a collected water tank 205 for recycling. Alternatively, the filtrated water is released to a natural setting.
In the filtration unit 203, since clogging of the filter F occurs, washing is carried out periodically. For example, a valve B1 connected to the raw water tank 201 is closed, a valve B3 and a valve B2, for delivering washing water from the raw water tank are opened, and the filter F is cleaned by a reverse flow of water from the collected water tank 205. The resultant wastewater containing a high concentration of Si waste is returned to the raw water tank 201. Also, the concentrated water in a concentrated water tank 206 is transported via a pump 208 to a centrifugal separator 209 and is thereby separated into sludge and separated fluid. The sludge comprising Si waste is collected into a sludge recovery tank 210 and the separated fluid is collected into a separated-fluid tank 211. After further accumulation of the separated fluid, the wastewater in the separated-fluid tank 211 is transported to the raw water tank 201 via a pump 212.
These methods have also been employed for the recovery of waste resulting from grinding or abrasion of a solid or plate-like member formed essentially from a metal material, such as Cu, Fe, Al, etc., or from grinding or abrasion of a solid or plate-like member formed from ceramic or other inorganic material.
Chemical-mechanical polishing (CMP) has come to be employed as a new semiconductor processing technology.
This CMP technique enables
(1): the realization of smooth device surface shapes; and
(2): the realization of structures with embedded materials that differ from the substrate.
With regard to (1) above, fine patterns are formed precisely using lithography techniques. The combined use of techniques for affixing Si wafers enables materialization of three-dimensional IC's.
With (2), embedded structures are made possible. Since priorly, a technique of embedding tungsten (W) has been employed in multilayer wiring of IC's. With this technique, W is embedded by a CVD method in a trench of an interlayer film and the surface is made smooth by etching back. However, smoothing by CMP has come to be employed recently. Other examples of application of this embedding technique include damascene processes and element separation.
Such CMP techniques and applications are described in detail in “Science of CMP,” published by Science Forum Co., Ltd.
A mechanism for a CMP process shall now be described briefly. As show in FIG. 18, a semiconductor wafer 252 is placed on an abrasive cloth 251 placed over a rotary table 250, and irregularities of the wafer 252 surface are eliminated by performing lapping, polishing, and chemical etching while pouring on an abrasive (slurry) 253. Smoothing is achieved by chemical reactions induced by a solvent included in the abrasive 253 and by mechanical abrasive actions of the abrasive cloth and the abrasive grains in the abrasive. Foamed polyurethane or non-woven fabric, etc., is used for example as the abrasive cloth 251. The abrasive has abrasive grains of silica, alumina, etc., mixed in water containing a pH regulator and is generally referred to as slurry. Lapping is performed while pouring on this slurry 253 and applying pressure onto the abrasive cloth 251 while rotating the wafer 252. 254 indicates a dressing part, which maintains the abrading ability of the abrasive cloth 251 and constantly keeps the surface of the abrasive cloth 251 in a dressed condition. M1, M2, and M3 indicate motors and 255 to 257 indicate belts.
The above-described mechanism is arranged as a system as shown for example in FIG. 19. This system largely comprises a wafer cassette loading/unloading station 260, wafer transfer mechanism part 261, the abrasive mechanism part 262, which was described using FIG. 12, a wafer cleaning mechanism part 263, and a system controller for controlling these parts.
A cassette 264 having wafers stored therein is placed in the wafer cassette loading/unloading station 260, and a wafer is taken out of the cassette 264. In the wafer transfer mechanism part 261, the wafer is retained, for example, by a manipulator 265, and is placed on the rotary table 250 disposed in the abrasive mechanism part 262. The wafer is then smoothed by means of the CMP technique. After smoothing of the wafer has been completed, the wafer is transported by means of a manipulator 266 to the wafer cleaning mechanism part 263 wherein the slurry is cleaned off of the wafer. The washed wafer is then housed in the wafer cassette 266.
The amount of slurry used for one abrasion process is about 500 cc to 1 liter/wafer. Also, pure water is made to flow in the above-described abrasive mechanism part 262 and the wafer cleaning mechanism part 263. Since the resulting wastewater are merged in the final stage at a drain, about 5 to 10 liters/wafer of wastewater flows out during a single smoothing operation. In the case of producing, for example, a three-layer-metal wafer, about seven smoothing operations are required for smoothing the metal and interlayer dielectric films. Thus wastewater of an amount of seven times the 5 to 10 liters is discharged for production of a single wafer.
It can thus be understood that the use of a CMP machine involves discharge of a considerable amount of slurry diluted with pure water.